The present invention relates, in general, to using a clamp circuit to protect a semiconductor device, and more particularly, to detecting the status of the clamp circuit.
Power semiconductor devices are used in high power applications such as automotive electronics, machine automation, and the like. For example, a power semiconductor device such as a power metal oxide semiconductor field effect transistor (MOSFET) is frequently used to switch an inductive load. When the power MOSFET is switched off, the inductive energy stored in the inductive load generates a surge in the drain voltage of the power MOSFET. A drain-gate clamp is typically employed to protect the power MOSFET from an avalanche stress induced failure that may be caused by such a voltage surge. The clamp diode has an avalanche voltage slightly less than that of the power MOSFET. When the drain-source voltage of the power MOSFET rises above the avalanche voltage of the clamp diode, a current flows through the clamp diode. The current activates the power MOSFET by developing a voltage across a resistor placed between the gate and source electrodes of the power MOSFET. The power MOSFET is turned on and dissipates all of the inductive energy in a less stressful conduction mode. A blocking diode is usually placed in series with the drain-gate clamp diode to block current flow from the gate to the drain of the power MOSFET.
Activation of the clamp circuit when switching off the power MOSFET indicates that the inductive load is intact. A malfunctioning inductive load, such as an open load or a shorted load, does not store inductive energy and, therefore, cannot activate the clamp circuit. Therefore, detecting the activation of the clamp circuit provides a diagnosis of the state of the inductive load as well as the state of the clamp circuit.
One approach for inferring the clamp circuit activation is to detect when the power MOSFET drain voltage exceeds a predetermined value. The approach uses a voltage divider to scale the drain voltage of the power MOSFET. The scaled drain voltage is transmitted to a comparator and compared with a reference voltage. The comparator generates an output signal when the scaled drain voltage exceeds the reference voltage. This approach does not directly detect the activation of the clamp circuit, but detects whether the drain voltage of the power MOSFET reaches a predetermined value. It is possible to have a drain voltage sufficient to generate the output signal but insufficient to activate the clamp circuit. It is also possible for the clamp circuit to be activated by a drain voltage that is insufficient to generate the output signal., Furthermore, because this approach is sensitive to the supply voltage level, the reference voltage may need to be adjusted whenever the supply voltage changes. In addition, the voltage divider is directly connected to the drain of the power MOSFET, where the voltage may reach a very high value when switching off the power MOSFET. The voltage divider also increases the drain leakage current. In a battery application where the load remains connected to the battery, any drain leakage is undesirable.
Accordingly, it would be advantageous to have a method for detecting the activation of a clamp circuit. It would be of further advantage for the method to be simple and reliable, and for the clamp circuit to have minimum current leakage and be suitable for manufacture using conventional manufacturing techniques.